The present invention relates to coated board comprising at least one polymer-based coat which prevents the penetration of liquids and gases. Furthermore, the invention comprises a process for manufacturing the board, and dishes and packaging, such as food packaging formed from the board
The board used for packaging liquid foods and other products sensitive to spoiling must, for the sake of the durability of the product, be impermeable to liquid and gas. Such packaging board prevents airborne oxygen from penetrating to the inside of the package and, consequently, the product from spoiling and, respectively, the aromas which evaporate from the product from escaping from the package. In board used for disposable containers, it is important to have a liquid-resistant coat which protects the board from wetting.
One known method for rendering product packaging impermeable to liquid and gas is to provide the board used for the packaging with a metal foil. However, the disadvantages of such packaging include high manufacturing costs, non-biodegradability of the foil layer, problems of regeneration of the packaging material, and the unsuitability of the packaging for heating in a microwave oven.
Because of the said problems associated with metal foil, a shift has been made in food packaging to increased use of boards in which the impermeability to liquid and gas has been achieved by means of one or more polymeric barrier layers. Among polymer materials, especially EVOH has excellent barrier properties but, for example, polyamide is also usable for the purpose. By combining various polymer materials, an impermeability substantially corresponding to that of aluminum foil has been achieved, but owing to the required successive barrier and bonding agent layers the board becomes complicated in structure and the consumption of polymer material is high.
The special problems of EVOH in packaging boards include its mechanical weakness and sensitivity to moisture. During the heat sealing of the board, the EVOH layer tends to be perforated by the steam escaping out of the board. In these respects polyamide is a material more durable than EVOH ,but its barrier properties are not equal to those of EVOH. There are known packaging boards in which the solution to the problem has been sought through a suitable combining of EVOH and polyamide. A common problem associated with on-line coating of boards with polymers continues to be the risk of sticking during the rolling subsequent to the drying of the coat. The sticking will damage the coat, which should specifically be unbroken and continuous in order to provide the desired impermeability to oxygen and water vapors. The large amount of polymeric material present in the coating layers also detract from the pulpability of the board during its recycling.
The object of the present invention is to provide a board provided with a polymer-based coat impermeable to liquids and gases, substantially avoiding the said disadvantages associated with known coated boards. Board according to the invention is characterized in that the coat is formed on the board during its manufacture from a polymer dispersion applied as on-line coating, to which dispersion talc particles have been added so that the talc will constitute. 30-80% of the total weight of the dried coat.
In the coated board according to the invention, impermeability to water, water vapor, oxygen and aromas can be achieved by means of suitable batching of talc, in addition to which the coat is impermeable to fats and oils. The barrier properties of the coat are based on the presence of a very pure talc made up substantially of small, flaky particles, typically less than 50 xcexcm in size, while the polymer closing the gaps between the talc particles forms a continuous phase which finally ensures the impermeability of the coat. It should be noted that calling the coat polymer-based refers to the presence of the. said continuous polymer phase and not to the polymer necessarily constituting the largest ingredient proportion in the coat, a fact which can be observed even from the range of the weight proportion of talc.
Besides the achievable barrier properties, the talc-containing coat of board also has many other advantages over known coating materials. The talc-containing coat is not sensitive to moisture; it protects the board from wetting so that the board retains its mechanical strength. The coating is thus especially suitable. for disposable containers and for packaging of liquid foodstuffs. The talc-containing coating compound is also suitable, without problems, for on-line coating processes which are in use, and the coating does not have a tendency to stick when the completed board is wound on a roll. The coating withstands heating, for which reason the coated board is suitable, for example, for baking containers. The coating also improves the mechanical properties of the board by increasing its stiffness.
Furthermore, the talc-containing coating has good printing properties owing to the fact that its surface is not hydrophobic. It is possible to carry out printing without a corona treatment, which is required by many other polymers, such as polyethylene, used as coatings.
Since the presence of talc in the coating compound reduces the amount of polymeric coating material, the coated board according to the invention is better pulpable and thus easier to recycle than known boards having corresponding barrier properties. For the same reason the use of talc is preferable in terms of the compostability and biodegrability of the board.
A talc-containing coat is in itself transparent, which maybe an advantage in some uses of the board. On the other hand, the talc-containing coating compound can be easily colored by adding pigments to the polymer dispersion forming the coat, applied onto the board.
The oxygen and water vapor barrier properties of a talc-containing coat depend on the basis weight of the coat and the amount of talc therein. Generally speaking, the dry weight of an individual talc-containing coat on either side of the board may vary within a range of 2-40 g/m2. Preferably the weight of the coat is 5-40 g/m2, by means of which it is possible to achieve impermeability of the coat to water vapor, and most preferably within a range of 8-20 g/m2, in which case the coat can be made impermeable to oxygen, a so-called High Barrier coating with an oxygen permeability below 100 cm3O2/m2xc2x7d. The proportion of talc is 30-80%, preferably 40-75%, of the weight of the dried coat.
Polymers suitable for the polymer basis of a coat according to the invention for a board include styrene butadiene, styrene acrylate, acrylate or vinyl acetate polymers and copolymers, or blends of these. The polymer may be prepared by using a monomer blend containing as its principal components vinyl acetate and a (methyl, ethyl, propyl or butyl) ester of acrylic acid and/or methacrylic acid and/or lower alcohols, or by using a monomer blend containing as its principal components styrene and a (methyl, ethyl, propyl or butyl) ester of acrylic acid and/or methacrylic acid and/or lower alcohols or by using a monomer blend containing as its principal components a (methyl, ethyl, propyl or butyl) ester of acrylic acid and/or methacrylic acid and/or lower alcohols and/or a copolymer of these. The said polymers form polymer latexes, i.e. polymer dispersions, which can be combined with talc particles and be applied onto board as a coat in which dispersed polymer particles join one another as a polymer phase which binds the talc particles together. Furthermore, polylactides, polyhydroxybutyrates/polyhydroxy-valerates, modified starches and other biopolymers which are compostable or entirely biodegradable can be mentioned as usable polymers which are especially advantageous.
The colorability, already pointed out above, of a talc-containing coat is an especially advantageous property, in particular in the packaging of foods which must be protected from the detrimental effect of ultraviolet light. The coat of board intended for food packaging may thus advantageously contain, in addition to talc, also some coloring pigment in an amount of at maximum 5% of the total coating layer. For example, soot, metal pigments, mineral pigments and organic pigments can be used.
In addition to talc, the coat may contain some other mineral component which serves as a filler. Examples of such components, the weight proportion of which is most preferably at maximum 30%, include titanium dioxide, calcium carbonate, kaolin and gypsum.
In a board to be used for food packaging and containers it may be preferable to incorporate into the coating compound a hydrophobic agent, for example, a wax dispersion such as paraffin wax, PE wax or a AKD wax dispersion, in an amount of at maximum 20% of the total weight of the coat. A hydrophobic coat, decreases the tendency of the food to adhere to the board. Wax, and a mineral pigment used together with it, additionally reduce the sticking together of the dried coats during rolling. A high wax content may, on the other hand, weaken the printing properties of the surface.
The forming of a coat on a board is preferably carried out by applying a talc-containing dispersion onto the board in two or more successive steps, the dispersion applied in the preceding step being dried before the subsequent application step. By such a step-wise coating procedure a coat of a better quality is achieved than by applying the entire amount of dispersion for the coat onto the board at one time.
Board intended for heat sealable packaging can, in addition to a talc-containing coat according to the invention, be provided with one or more polymeric heat sealing layers adhering to the coat. There may thus be a talc-containing coat on one side of the board and a heat sealing layer sealable to the said coat on the opposite side of the board. Alternatively, both sides of the board may be provided with a heat sealing layer, in which case at least on one side of the board the heat sealing layer is applied on top of a talc-containing coat. Suitable materials for the heat sealing layer include LD-polyethylene and heat sealing lacquers.
Since the heat sealing layer, contrary to the talc-containing coat, may in order to be printable require a corona treatment, it may be advantageous to introduce the heat sealing polymer onto the board only in narrow streaks in the areas of the heat seals to be produced. Thus the said polymer will not complicate the printing of the areas between the seals. At the same time, savings of material are achieved and the structure of the board and the products made therefrom is rendered lighter.
A board according to the invention, provided with a talc-containing polymer coat, is most preferably a multi-layer board comprising two or more fiber-based layers; its weight without the coats may be within a range of 130-500 g/m2, preferably 170-350 g/m2. One example is a three-layer board which comprises a thicker middle layer which is formed in part or entirely from a mechanical pulp, such as CTMP; on both sides of the middle layer there are thinner outer layers formed from a sulfate pulp. Onto the outer layers on both sides of the board there are introduced polymeric coats which, in accordance with the invention, include at least one talc-containing barrier layer. The said board in itself has a stiff and non-buckling structure, and the talc-containing coat according to the invention on one or both sides of the board gives the board additional stiffness. The talc-containing barrier provides the further advantage that the ingredients of wood origin present in. the mechanical pulp will not give detrimental odor or taste to the packaged product. If recycled paper pulp is used for making the board, the barrier layers correspondingly insulate the product from any impurities present in the board.
The process according to the invention for the manufacture of the coated board described above is characterized in that, in connection with the manufacturing process, there is applied onto the board as on-line coating a polymer dispersion to which talc particles are added so that in the completed board talc will constitute 30-80% of the total weight of the dried coat.
Of packaging according to the present invention, made from board in accordance with the above, there must be mentioned, above all, food packaging in which the talc-containing barrier layers of the board ensure the durability of the product. Products packaged in accordance with the invention are protected from airborne oxygen and from outside moisture, while the water, fat and aroma barrier properties of the coat preserve the quality of the product and prevent packaged products from damaging one another during storage. Examples of products to be packaged include moist or liquid foods, such as juices, water, milk and other milk products such as cream, buttermilk, yogurt and ice-cream. Also possible are dry foods such as flours, powders, breakfast cereals, and animal foods. For example, the inside plastic bags so far used in breakfast cereal packages will be unnecessary.
The board can be used not only for industrial product packaging but also for manufacturing various containers, such as disposable tableware. The shape of the package or container is not limited; cases, cartons, tubs, baking containers, cups, mugs, goblets, trays, plates, etc., are possible.
An especially advantageous area of application for products according to the invention consists of board for frozen-foods containers, in which an inside talc-containing coat prevents the product from sticking to the container. Examples of frozen products to be packaged include ready-made foods such as casseroles and ice-cream.
The packaging of products, other than foods requiring shielding against the transmission of oxygen, moisture and aromas may also come within the scope of the invention. One example is cigarette boxes in which the talc-containing coat of the packaging board renders unnecessary. the aluminum foil shield used inside the boxes to date.
Uses outside the packaging field for coated board according to the invention further include post-cards and advertising posters. In the latter, coats which protect the board layer from wetting are a considerable advantage.